Do Möbius Silabenzene and Möbius Phosphabenzene Exist? Ab Initio MO and Density Functional Study of Electrocyclic Ring-Opening Reactions of Hetero-Dewar Benzenes Containing Silicon or Phosphorus

Ab initio molecular orbital and hybrid density functional calculations have been performed
on the electrocyclic ring opening of cis-Dewar silabenzenes 1a and 2a and cis-Dewar
phosphabenzenes 1b and 2b. Complete structural optimizations at the correlated MP2/
6-31G*, CCSD(T)/6-31G*//MP2/6-31G*, B3LYP/6-31G*, and B3LYP//6-311G*/B3LYP/6-31G*
levels predict the existence of Möbius silabenzene (3a; 1,3,5-cis,cis,trans-1-silacyclohexatriene), having an energy 70−76 kcal mol-1 higher than that of silabenzene 4a. Möbius
phosphabenzene (3b; 1,3,5-cis,cis,trans-1-phosphacyclohexatriene) is predicted to exist 80−85 kcal mol-1 above phosphabenzene 4b. Ring opening of cis-Dewar silabenzene 1a involves
a barrier of 20−23 kcal mol-1 and results in the formation of Möbius silabenzene (3a). Ring
opening of 1b produces the analogous Möbius phosphabenzene (3b) by overcoming a barrier
of 26−31 kcal mol-1. These potential energy barriers are very close to the experimental
enthalpic barrier of 23−25 kcal mol-1 for the formation of the parent Möbius benzene. The
structures of 3a and 3b exhibit one fully trans double bond with dihedral angle H−C−C−H
≈ 180°. Bonding considerations reveal that Möbius silabenzene is antiaromatic, while Möbius
phosphabenzene is nonaromatic in nature. The rotation of the trans π-bond in 3a and 3b
results in the lowest energy benzenoid isomers 4a and 4b. Calculations also support the
existence of higher energy trans-Dewar silabenzene (5a) and trans-Dewar phosphabenzene
(5b) as potential minima. The ring opening of 5a yields silabenzene, while that of 5b results
in phosphabenzvalene 6b. We also characterized two other higher energy phosphabenzvalene isomers, 7b and 8b, as potential minima. The phosphabenzvalene 6b is found to be
closer in energy to the benzenoid form 4b than the Dewar phosphabenzene isomers
investigated in this study. The present analysis reveals that a conrotatory mode of ring
opening takes place in Dewar silabenzenes and Dewar phosphabenzenes. Progress of the
electrocyclic ring-opening reactions is monitored by estimating the extent of bond formation/breakage in the respective transition states. It is found that the extent of π-bond breaking
is more pronounced than that of π-bond formation at the transition states.